Electrophysiological and morphological properties of pre-autonomic neurones in the rat hypothalamic paraventricular nucleus

Exercise-induced neuronal plasticity in central autonomic networks: role in cardiovascular control

It is now well established that brain plasticity is an inherent property not only of the developing but also of the adult brain. Numerous beneficial effects of exercise, including improved memory, cognitive function and neuroprotection, have been shown to involve an important neuroplastic component. However, whether major adaptive cardiovascular adjustments during exercise, needed to ensure proper blood perfusion of peripheral tissues, also require brain neuroplasticity, is presently unknown. This review will critically evaluate current knowledge on proposed mechanisms that are likely to underlie the continuous resetting of baroreflex control of heart rate during/after exercise and following exercise training. Accumulating evidence indicates that not only somatosensory afferents (conveyed by skeletal muscle receptors, baroreceptors and/or cardiopulmonary receptors) but also projections arising from central command neurons (in particular, peptidergic hypothalamic pre-autonomic neurons) converge into the nucleus tractus solitarii (NTS) in the dorsal brainstem, to co-ordinate complex cardiovascular adaptations during dynamic exercise. This review focuses in particular on a reciprocally interconnected network between the NTS and the hypothalamic paraventricular nucleus (PVN), which is proposed to act as a pivotal anatomical and functional substrate underlying integrative feedforward and feedback cardiovascular adjustments during exercise. Recent findings supporting neuroplastic adaptive changes within the NTS-PVN reciprocal network (e.g. remodelling of afferent inputs, structural and functional neuronal plasticity and changes in neurotransmitter content) will be discussed within the context of their role as important underlying cellular mechanisms supporting the tonic activation and improved efficacy of these central pathways in response to circulatory demand at rest and during exercise, both in sedentary and in trained individuals. We hope this review will stimulate more comprehensive studies aimed at understanding cellular and molecular mechanisms within CNS neuronal networks that contribute to exercise-induced neuroplasticity and cardiovascular adjustments.

Enhanced angiotensin-mediated excitation of renal sympathetic nerve activity within the paraventricular nucleus of anesthetized rats with heart failure

Chronic heart failure (HF) is characterized by increased sympathetic drive. Enhanced angiotensin II (ANG II) activity may contribute to the increased sympathoexcitation under HF condition. The present study examined sympathoexcitation by 1) the effects of ANG II in the paraventricular nucleus (PVN) on renal sympathetic nerve activity (RSNA), and 2) the altered ANG II type 1 (AT(1)) receptor expression during HF. Left coronary artery ligation was used to induce HF. In the anesthetized Sprague-Dawley rats, microinjection of ANG II (0.05-1 nmol) into the PVN increased RSNA, mean arterial pressure (MAP), and heart rate (HR) in both sham-operated and HF rats. The responses of RSNA and HR were significantly enhanced in rats with HF compared with sham rats (RSNA: 64 +/- 8% vs. 33 +/- 4%, P < 0.05). Microinjection of AT(1) receptor antagonist losartan into the PVN produced a decrease of RSNA, MAP, and HR in both sham and HF rats. The RSNA and HR responses to losartan in HF rats were significantly greater (RSNA: -25 +/- 4% vs. -13 +/- 1%, P < 0.05). Using RT-PCR and Western blot analysis, we found that there were significant increases in the AT(1) receptor mRNA (Delta186 +/- 39%) and protein levels (Delta88 +/- 20%) in the PVN of rats with HF (P < 0.05). The immunofluorescence of AT(1) receptors was significantly higher in the PVN of rats with HF. These data support the conclusion that an increased angiotensinergic activity on sympathetic regulation, due to the upregulation of ANG II AT(1) receptors within the PVN, may contribute to the elevated sympathoexcitation that is observed during HF.

Excitability of paraventricular nucleus neurones that project to the rostral ventrolateral medulla is regulated by small-conductance Ca2+-activated K+ channels

Whole cell patch-clamp recordings were performed in brain slices to investigate mechanisms regulating the excitability of paraventricular nucleus (PVN) neurones that project directly to the rostral ventrolateral medulla (RVLM) (PVN-RVLM neurones) of rats. In voltage-clamp recordings, step depolarization elicited a calcium-dependent outward tail current that reversed near E(K). The current was nearly abolished by apamin and by UCL1684, suggesting mediation by small-conductance Ca(2+)-activated K(+) (SK) channels. In current-clamp recordings, depolarizing step current injections evoked action potentials that underwent spike-frequency adaptation (SFA). SK channel blockade with apamin or UCL1684 increased the spike frequency without changing the rate of SFA. Upon termination of step current injection, a prominent medium after-hyperpolarization potential (mAHP) was observed. SK channel blockade abolished the mAHP and revealed an after-depolarization potential (ADP). In response to ramp current injections, the rate of sub-threshold depolarization was increased during SK channel blockade, indicating that depolarizing input resistance was increased. Miniature EPSC frequency, amplitude, and decay kinetics were unaltered by bath application of apamin, suggesting that SK channel blockade likely increased excitability by a postsynaptic action. We conclude that although SK channels play little role in generating SFA in PVN-RVLM neurones, their activation nevertheless does dampen excitability. The mechanism appears to involve activation of a mAHP that opposes a prominent ADP that would otherwise facilitate firing.

C-peptide of preproinsulin-like peptide 7: localization in the rat brain and activity in vitro

With the use of a rabbit polyclonal antiserum against a conserved region (54-118) of C-peptide of human preproinsulin-like peptide 7, referred to herein as C-INSL7, neurons expressing C-INSL7-immunoreactivity (irC-INSL7) were detected in the pontine nucleus incertus, the lateral or ventrolateral periaqueductal gray, dorsal raphe nuclei and dorsal substantia nigra. Immunoreactive fibers were present in numerous forebrain areas, with a high density in the septum, hypothalamus and thalamus. Pre-absorption of C-INSL7 antiserum with the peptide C-INSL7 (1 microg/ml), but not the insulin-like peptide 7 (INSL7; 1 microg/ml), also known as relaxin 3, abolished the immunoreactivity. Optical imaging with a voltage-sensitive dye bis-[1,3-dibutylbarbituric acid] trimethineoxonol (DiSBAC4(3)) showed that C-INSL7 (100 nM) depolarized or hyperpolarized a small population of cultured rat hypothalamic neurons studied. Ratiometric imaging studies with calcium-sensitive dye fura-2 showed that C-INSL7 (10-1000 nM) produced a dose-dependent increase in cytosolic calcium concentrations [Ca2+]i in cultured hypothalamic neurons with two distinct patterns: (1) a sustained elevation lasting for minutes; and (2) a fast, transitory rise followed by oscillations. In a Ca2+-free Hanks' solution, C-INSL7 again elicited two types of calcium transients: (1) a fast, transitory increase not followed by a plateau phase, and (2) a transitory rise followed by oscillations. INSL7 (100 nM) elicited a depolarization or hyperpolarization in a small population of hypothalamic neurons, and an increase of [Ca2+]i with two patterns that were dissimilar from that of C-INSL7. [125I]C-INSL7 bindings to rat brain membranes were inhibited by C-INSL7 in a dose-dependent manner; the Kd and Bmax. values were 17.7 +/- 8.2 nM and 45.4 +/- 20.5 fmol/mg protein. INSL7 did not inhibit [125I]C-INSL7 binding to rat brain membranes, indicating that C-INSL7 and INSL7 bind to distinct binding sites. Collectively, our result raises the possibility that C-INSL7 acts as a signaling molecule independent from INSL7 in the rat CNS.

Adiponectin depolarizes parvocellular paraventricular nucleus neurons controlling neuroendocrine and autonomic function

Adiponectin plays important roles in the control of energy homeostasis and autonomic function through peripheral and central nervous system actions. The paraventricular nucleus (PVN) of the hypothalamus is a primary site of neuroendocrine (NE) and autonomic integration, and, thus, a potential target for adiponectin actions. Here, we investigate actions of adiponectin on parvocellular PVN neurons. Adiponectin influenced the majority (65%) of parvocellular PVN neurons, depolarizing 47%, whereas hyperpolarizing 18% of neurons tested. Post hoc identification (single-cell RT-PCR) after recordings revealed that adiponectin depolarizes NE-CRH neurons, whereas intracerebroventricular injections of adiponectin in vivo caused increased plasma ACTH concentrations. Adiponectin also depolarized the majority of TRH neurons, however, NE-TRH neurons were unaffected, in accordance with in vivo experiments showing that intracerebroventricular adiponectin was without effect on plasma TSH. In addition, bath administration of adiponectin also depolarized both preautonomic TRH and oxytocin neurons. These results show that adiponectin acts in the central nervous system to coordinate NE and autonomic function through actions on specific functional groups of PVN neurons.

Intra-carotid hyperosmotic stimulation increases Fos staining in forebrain organum vasculosum laminae terminalis neurones that project to the hypothalamic paraventricular nucleus

Body fluid hyperosmolality has long been known to elicit homeostatic responses that range from drinking to inhibition of salt appetite to release of neurohypohyseal hormones (i.e. vasopressin and oxytocin). More recently, it has been recognized that hyperosmolality is capable of also provoking a significant increase of sympathetic nerve activity (SNA). It has been reported that neurones in the forebrain organum vasculosum laminae terminalis (OVLT) and hypothalamic paraventricular nucleus (PVN) each contribute significantly to this response. Here we sought to determine if sympathoexcitatory levels of hyperosmolality activate specifically those OVLT neurones that form a monosynaptic pathway to the PVN. First, we established in anaesthetized rats that graded concentrations of hypertonic NaCl (1.5 and 3.0 osmol kg(-1)) elicit graded increases of renal SNA (RSNA) when infused at a rate of 0.1 ml min(-1) through an internal carotid artery (ICA) - the major vascular supply of the forebrain. Next, infusions were performed in conscious rats in which OVLT neurones projecting to the PVN (OVLT-PVN) were retrogradely labelled with cholera toxin subunit B (CTB). Immunostaining of the immediate early gene product Fos and CTB was performed to quantify osmotic activation of OVLT-PVN neurones. ICA infusions of hypertonic NaCl and mannitol each significantly (P < 0.01-0.001) increased the number of Fos immunoreactive (Fos-ir) neuronal nuclei in the dorsal cap (DC) and lateral margins (LM) of OVLT. In the LM, infusions of 1.5 and 3.0 osmol kg(-1) NaCl produced similar increases in the number of Fos-ir neurones. In the DC, these infusions produced graded increases in Fos expression. Among OVLT neurones with axons projecting directly to the PVN (i.e. CTB-ir), graded hypertonic NaCl infusions again produced graded increases in Fos expression and this was observed in both the DC and LM. Although the DC and LM contained a similar number of OVLT-PVN neurones, the proportion of such neurones that expressed Fos-ir in responses to ICA hypertonic NaCl infusions was greater in the DC (P < 0.001). These findings support the conclusion that PVN-projecting neurones in the DC and LM of OVLT could participate in behavioural, neuroendocrine, and sympathetic nervous system responses to body fluid hyperosmolality.

Distribution of glucagon-like peptide-1 immunoreactivity in the hypothalamic paraventricular and supraoptic nuclei

Glucagon-like peptide-1 (GLP-1) plays a role in modulating neuroendocrine and autonomic function. The hypothalamic paraventricular nucleus (PVN) contains aggregations of GLP-1 fibers and expresses GLP-1 receptors, making it a likely site of action for GLP-1 signaling. The current study was designed to establish domains of GLP-1 action, focusing on axosomatic appositions on different neuroendocrine and autonomic cell populations in the PVN. The data indicate abundant GLP-1-immunoreactive terminal appositions on corticotropin-releasing hormone neurons in the medial parvocellular PVN. GLP-1 positive boutons can also be observed in apposition to oxytocinergic neurons and on retrogradely labeled pre-autonomic neurons projecting to the region of the nucleus of the solitary tract. In contrast, there were very few vasopressinergic neurons with GLP-1 appositions. Overall, the data indicate that the central GLP-1 system preferentially targets neurons in hypophysiotrophic zones of the PVN, consistent with excitatory actions of GLP-1 on adrenocorticotropin release. GLP-1 is also in position to influence oxytocin secretion and control outflow to brainstem cardiovascular relays.

Heterogeneous distribution of basal cyclic guanosine monophosphate within distinct neuronal populations in the hypothalamic paraventricular nucleus

The supraoptic (SON) and the paraventricular (PVN) hypothalamic nuclei constitute major neuronal substrates underlying nitric oxide (NO) effects on autonomic and neuroendocrine control. Within these nuclei, constitutively produced NO restrains the firing activity of magnocellular neurosecretory and preautonomic neurons, actions thought to be mediated by a cGMP-dependent enhancement of GABAergic inhibitory transmission. In the present study, we expanded on this knowledge by performing a detailed anatomical characterization of constitutive NO-receptive, cGMP-producing neurons within the PVN. To this end, we combined tract-tracing techniques and immunohistochemistry to visualize cGMP immunoreactivity within functionally, neurochemically, and topographically discrete PVN neuronal populations in Wistar rats. Basal cGMP immunoreactivity was readily observed in the PVN, both in neuronal and vascular profiles. The incidence of cGMP immunoreactivity was significantly higher in magnocellular (69%) compared with preautonomic ( approximately 10%) neuronal populations (P < 0.01). No differences were observed between oxytocin (OT) and vasopressin (VP) magnocellular neurons. In preautonomic neurons, the incidence of cGMP was independent of their subnuclei distribution, innervated target (i.e., intermediolateral cell column, nucleus tractus solitarii, or rostral ventrolateral medulla) or their neurochemical phenotype (i.e., OT or VP). Finally, high levels of cGMP immunoreactivity were observed in GABAergic somata and terminals within the PVN of eGFP-GAD67 transgenic mice. Altogether, these data support a highly heterogeneous distribution of basal cGMP levels within the PVN and further support the notion that constitutive NO actions in the PVN involve intricate cell-cell interactions, as well as heterogeneous signaling modalities.

Molecular characterization of T-type Ca(2+) channels responsible for low threshold spikes in hypothalamic paraventricular nucleus neurons

The hypothalamic paraventricular nucleus (PVN) is composed of functionally heterogeneous cell groups, possessing distinct electrophysiological properties depending on their functional roles. Previously, T-type Ca(2+) dependent low-threshold spikes (LTS) have been demonstrated in various PVN neuronal types, including preautonomic cells. However, the molecular composition and functional properties of the underlying T-type Ca(2+) channels have not been characterized. In the present study, we combined single cell reverse transcription-polymerase chain reaction (RT-PCR), immunohistochemistry and patch-clamp recordings to identify subtypes of T-type Ca(2+) channels expressed in PVN cells displaying LTS (PVN-LTS), including identified preautonomic neurons. LTS appeared at the end of hyperpolarizing pulses either as long-lasting plateaus or as short-lasting depolarizing humps. LTS were mediated by rapidly activating and inactivating T-type Ca(2+) currents and were blocked by Ni(2+). Single cell RT-PCR and immunohistochemical studies revealed Cav3.1 (voltage-gated Ca(2+) channel) as the main channel subunit detected in PVN-LTS neurons. In conclusion, these data indicate that Cav3.1 is the major subtype of T-type Ca(2+) channel subunit that mediates T-type Ca(2+) dependent LTS in PVN neurons.

Right atrial stretch alters fore- and hind-brain expression of c-fos and inhibits the rapid onset of salt appetite

The inflation of an intravascular balloon positioned at the superior vena cava and right atrial junction (SVC-RAJ) reduces sodium or water intake induced by various experimental procedures (e.g. sodium depletion; hypovolaemia). In the present study we investigated if the stretch induced by a balloon at this site inhibits a rapid onset salt appetite, and if this procedure modifies the pattern of immunohistochemical labelling for Fos protein (Fos-ir) in the brain. Male Sprague-Dawley rats with SVC-RAJ balloons received a combined treatment of furosemide (Furo; 10 mg (kg bw)(-1)) plus a low dose of the angiotensin-converting enzyme inhibitor captopril (Cap; 5 mg (kg bw)(-1)). Balloon inflation greatly decreased the intake of 0.3 m NaCl for as long as the balloon was inflated. Balloon inflation over a 3 h period following Furo-Cap treatment decreased Fos-ir in the organum vasculosum of the lamina terminalis and the subfornical organ and increased Fos-ir in the lateral parabrachial nucleus and caudal ventrolateral medulla. The effect of balloon inflation was specific for sodium intake because it did not affect the drinking of diluted sweetened condensed milk. Balloon inflation and deflation also did not acutely change mean arterial pressure. These results suggest that activity in forebrain circumventricular organs and in hindbrain putative body fluid/cardiovascular regulatory regions is affected by loading low pressure mechanoreceptors at the SVC-RAJ, a manipulation that also attenuates salt appetite.

Developmental switch in neuropeptide Y and melanocortin effects in the paraventricular nucleus of the hypothalamus

Homeostatic regulation of energy balance in rodents changes dramatically during the first 3 postnatal weeks. Neuropeptide Y (NPY) and melanocortin neurons in the arcuate nucleus, a primary energy homeostatic center in adults, do not fully innervate the paraventricular nucleus (PVN) until the third postnatal week. We have identified two classes of PVN neurons responsive to these neuropeptides, tonically firing neurosecretory (NS) and burst-firing preautonomic (PA) cells. In neonates, NPY could inhibit GABAergic inputs to nearly all NS and PA neurons, while melanocortin regulation was minimal. However, there was a dramatic, age-dependent decrease in NPY responses specifically in the PA neurons, and a 3-fold increase in melanocortin responses in NS cells. These age-dependent changes were accompanied by changes in spontaneous GABAergic currents onto these neurons. This primarily NPYergic regulation in the neonates likely promotes the positive energy balance necessary for growth, while the developmental switch correlates with maturation of homeostatic regulation of energy balance.

Diminished A-type potassium current and altered firing properties in presympathetic PVN neurones in renovascular hypertensive rats

Accumulating evidence supports a contribution of the hypothalamic paraventricular nucleus (PVN) to sympathoexcitation and elevated blood pressure in renovascular hypertension. However, the underlying mechanisms resulting in altered neuronal function in hypertensive rats remain largely unknown. Here, we aimed to address whether the transient outward potassium current (I(A)) in identified rostral ventrolateral medulla (RVLM)-projecting PVN neurones is altered in hypertensive rats, and whether such changes affected single and repetitive action potential properties and associated changes in intracellular Ca(2+) levels. Patch-clamp recordings obtained from PVN-RVLM neurons showed a reduction in I(A) current magnitude and single channel conductance, and an enhanced steady-state current inactivation in hypertensive rats. Morphometric reconstructions of intracellularly labelled PVN-RVLM neurons showed a diminished dendritic surface area in hypertensive rats. Consistent with a diminished I(A) availability, action potentials in PVN-RVLM neurons in hypertensive rats were broader, decayed more slowly, and were less sensitive to the K(+) channel blocker 4-aminopyridine. Simultaneous patch clamp recordings and confocal Ca(2+) imaging demonstrated enhanced action potential-evoked intracellular Ca(2+) transients in hypertensive rats. Finally, spike broadening during repetitive firing discharge was enhanced in PVN-RVLM neurons from hypertensive rats. Altogether, our results indicate that diminished I(A) availability constitutes a contributing mechanism underlying aberrant central neuronal function in renovascular hypertension.

Functional role of A-type potassium currents in rat presympathetic PVN neurones

Despite the fact that paraventricular nucleus (PVN) neurones innervating the rostral ventrolateral medulla (RVLM) play important roles in the control of sympathetic function both in physiological and pathological conditions, the precise mechanisms controlling their activity are still incompletely understood. In the present study, we evaluated whether the transient outward potassium current I(A) is expressed in PVN-RVLM neurones, characterized its biophysical and pharmacological properties, and determined its role in shaping action potentials and firing discharge in these neurones. Patch-clamp recordings obtained from retrogradely labelled, PVN-RVLM neurones indicate that a 4-AP sensitive, TEA insensitive current, with biophysical properties consistent with I(A), is present in these neurones. Pharmacological blockade of I(A) depolarized resting V(m) and prolonged Na(+) action potential duration, by increasing its width and by slowing down its decay time course. Interestingly, blockade of I(A) either increased or decreased the firing activity of PVN-RVLM neurones, supporting the presence of subsets of PVN-RVLM neurones differentially modulated by I(A). In all cases, the effects of I(A) on firing activity were prevented by a broad spectrum Ca(2+) channel blocker. Immunohistochemical studies suggest that I(A) in PVN-RVLM neurons is mediated by Kv1.4 and/or Kv4.3 channel subunits. Overall, our results demonstrate the presence of I(A) in PVN-RVLM neurones, which actively modulates their action potential waveform and firing activity. These studies support I(A) as an important intrinsic mechanism controlling neuronal excitability in this central presympathetic neuronal population.

A-type potassium channels differentially tune afferent pathways from rat solitary tract nucleus to caudal ventrolateral medulla or paraventricular hypothalamus

The solitary tract nucleus (NTS) conveys visceral information to diverse central networks involved in homeostatic regulation. Although afferent information content arriving at various CNS sites varies substantially, little is known about the contribution of processing within the NTS to these differences. Using retrograde dyes to identify specific NTS projection neurons, we recently reported that solitary tract (ST) afferents directly contact NTS neurons projecting to caudal ventrolateral medulla (CVLM) but largely only indirectly contact neurons projecting to the hypothalamic paraventricular nucleus (PVN). Since intrinsic properties impact information transmission, here we evaluated potassium channel expression and somatodendritic morphology of projection neurons and their relation to afferent information output directed to PVN or CVLM pathways. In slices, tracer-identified projection neurons were classified as directly or indirectly (polysynaptically) coupled to ST afferents by EPSC latency characteristics (directly coupled, jitter < 200 micros). In each neuron, voltage-dependent potassium currents (IK) were evaluated and, in representative neurons, biocytin-filled structures were quantified. Both CVLM- and PVN-projecting neurons had similar, tetraethylammonium-sensitive IK. However, only PVN-projecting NTS neurons displayed large transient, 4-aminopyridine-sensitive, A-type currents (IKA). PVN-projecting neurons had larger cell bodies with more elaborate dendritic morphology than CVLM-projecting neurons. ST shocks faithfully (> 75%) triggered action potentials in CVLM-projecting neurons but spike output was uniformly low (< 20%) in PVN-projecting neurons. Pre-conditioning hyperpolarization removed IKA inactivation and attenuated ST-evoked spike generation along PVN but not CVLM pathways. Thus, multiple differences in structure, organization, synaptic transmission and ion channel expression tune the overall fidelity of afferent signals that reach these destinations.

Prokineticin 2 depolarizes paraventricular nucleus magnocellular and parvocellular neurons

Blind whole-cell patch-clamp techniques were used to examine the effects of prokineticin 2 (PK2) on the excitability of magnocellular (MNC), parvocellular preautonomic (PA), and parvocellular neuroendocrine (NE) neurons within the hypothalamic paraventricular nucleus (PVN) of the rat. The majority of MNC neurons (76%) depolarized in response to 10 nm PK2, effects that were eliminated in the presence of tetrodotoxin (TTX). PK2 also caused an increase in excitatory postsynaptic potential (EPSP) frequency, a finding that was confirmed by voltage clamp recordings demonstrating increases in excitatory postsynaptic current (EPSC) frequency. The depolarizing effects of PK2 on MNC neurons were also abolished by kynurenic acid (KA), supporting the conclusion that the effects of PK2 are mediated by the activation of glutamate interneurons within the hypothalamic slice. PA (68%) and NE (67%) parvocellular neurons also depolarized in response to 10 nm PK2. However, in contrast to MNC neurons, these effects were maintained in TTX, indicating that PK2 directly affects PA and NE neurons. PK2-induced depolarizations observed in PA and NE neurons were found to be concentration-related and receptor mediated, as experiments performed in the presence of A1MPK1 (a PK2 receptor antagonist) abolished the effects of PK2 on these subpopulations of neurons. The depolarizing effects of PK2 on PA and NE neurons were also shown to be abolished by PD 98059 (a mitogen activated protein kinase (MAPK) inhibitor) suggesting that PK2 depolarizes PVN parvocellular neurons through a MAPK signalling mechanism. In combination, these studies have identified separate cellular mechanisms through which PK2 influences the excitability of different subpopulations of PVN neurons.

Exercise training-induced remodeling of paraventricular nucleus (nor)adrenergic innervation in normotensive and hypertensive rats

Activation of oxytocin (OT)ergic projections from the hypothalamic paraventricular nucleus (PVN) to the nucleus tractus solitarii contributes to cardiovascular adjustments during exercise training (EXT). Moreover, a deficit in this central OTergic pathway is associated with altered cardiovascular function in hypertension. Since PVN catecholaminergic inputs, known to be activated during EXT, modulate PVN cardiovascular-related functions, we aimed here to determine whether remodeling of PVN (nor)adrenergic innervation occurs during EXT and whether this phenomenon is affected by hypertension. Confocal immunofluorescence microscopy and tract tracing were used to quantify changes in (nor)adrenergic innervation density in PVN subnuclei and in identified dorsal vagal complex (DVC) projecting neurons (PVN-DVC) in EXT normotensive [Wistar-Kyoto rat (WKY)] and hypertensive [spontaneously hypertensive rat (SHR)] rats. In WKY, EXT increased the density of PVN dopamine β-hydroxylase immunoreactivity (DBHir) (160%). Furthermore, the number and density of DBHir boutons overlapping PVN-DVC OTergic neurons were also increased during EXT (130%), effects that were blunted in SHR. Conversely, while DBHir in the medial parvocellular subnucleus (an area enriched in corticotropin-releasing hormone neurons) was not changed by EXT in WKY, a diminished DBHir was observed in trained SHR. Overall, these data support the concept that the PVN (nor)adrenergic innervation undergoes plastic remodeling during EXT, an effect that is differentially affected during hypertension. The functional implications of PVN (nor)adrenergic remodeling in relation to the central peptidergic control of cardiovascular function during EXT are discussed.

Cranial visceral afferent pathways through the nucleus of the solitary tract to caudal ventrolateral medulla or paraventricular hypothalamus: target-specific synaptic reliability and convergence patterns

Cranial visceral afferents activate central pathways that mediate systemic homeostatic processes. Afferent information arrives in the brainstem nucleus of the solitary tract (NTS) and is relayed to other CNS sites for integration into autonomic responses and complex behaviors. Little is known about the organization or nature of processing within NTS. We injected fluorescent retrograde tracers into two nuclei to identify neurons that project to sites involved in autonomic regulation: the caudal ventrolateral medulla (CVLM) or paraventricular nucleus of the hypothalamus (PVN). We found distinct differences in synaptic connections and performance in the afferent path through NTS to these neurons. Anatomical studies using confocal and electron microscopy found prominent, primary afferent synapses directly on somata and dendrites of CVLM-projecting NTS neurons identifying them as second-order neurons. In brainstem slices, afferent activation evoked large, constant latency EPSCs in CVLM-projecting NTS neurons that were consistent with the precise timing and rare failures of monosynaptic contacts on second-order neurons. In contrast, most PVN-projecting NTS neurons lacked direct afferent input and responded to afferent stimuli with highly variable, intermittently failing synaptic responses, indicating polysynaptic pathways to higher-order neurons. The afferent-evoked EPSCs in most PVN-projecting NTS neurons were smaller and unreliable but also often included multiple, convergent polysynaptic responses not observed in CVLM-projecting neurons. A few PVN-projecting NTS neurons had monosynaptic EPSC characteristics. Together, we found that cranial visceral afferent pathways are structured distinctly within NTS depending on the projection target. Such, intra-NTS pathway architecture will substantially impact performance of autonomic or neuroendocrine reflex arcs.

Interaction between glutamate and GABA systems in the integration of sympathetic outflow by the paraventricular nucleus of the hypothalamus

The paraventricular nucleus (PVN) of the hypothalamus is a central site known to modulate sympathetic outflow. Excitatory and inhibitory neurotransmitters within the PVN dictate final outflow. The goal of the present study was to examine the role of the interaction between the excitatory neurotransmitter glutamate and the inhibitory neurotransmitter GABA in the regulation of sympathetic activity. In alpha-chloralose- and urethane-anesthetized rats, microinjection of glutamate and N-methyl-D-aspartate (NMDA; 50, 100, and 200 pmol) into the PVN produced dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate. These responses were blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP-5). Microinjection of bicuculline, a GABA(A) receptor antagonist, into the PVN (50, 100, and 200 pmol) also produced significant, dose-dependent increases in renal sympathetic nerve activity, blood pressure, and heart rate; AP-5 also blocked these responses. Using microdialysis and HPLC/electrochemical detection techniques, we observed that bicuculline infusion into the PVN increased glutamate release. Using an in vitro hypothalamic slice preparation, we found that bicuculline increased the frequency of glutamate-mediated excitatory postsynaptic currents in PVN-rostral ventrolateral medullary projecting neurons, supporting a GABA(A)-mediated tonic inhibition of this excitatory input into these neurons. Together, these data indicate that 1) glutamate, via NMDA receptors, excites the presympathetic neurons within the PVN and increases sympathetic outflow and 2) this glutamate excitatory input is tonically inhibited by a GABA(A)-mediated mechanism.

Opposing crosstalk between leptin and glucocorticoids rapidly modulates synaptic excitation via endocannabinoid release

The hypothalamic paraventricular nucleus (PVN) integrates preautonomic and neuroendocrine control of energy homeostasis, fluid balance, and the stress response. We recently demonstrated that glucocorticoids act via a membrane receptor to rapidly cause endocannabinoid-mediated suppression of synaptic excitation in PVN neurosecretory neurons. Leptin, a major signal of nutritional state, suppresses CB(1) cannabinoid receptor-dependent hyperphagia (increased appetite) in fasting animals by reducing hypothalamic levels of endocannabinoids. Here we show that glucocorticoids stimulate endocannabinoid biosynthesis and release via a Galpha(s)-cAMP-protein kinase A-dependent mechanism and that leptin blocks glucocorticoid-induced endocannabinoid biosynthesis and suppression of excitation in the PVN via a phosphodiesterase-3B-mediated reduction in intracellular cAMP levels. We demonstrate this rapid hormonal interaction in both PVN magnocellular and parvocellular neurosecretory cells. Leptin blockade of the glucocorticoid-induced, endocannabinoid-mediated suppression of excitation was absent in leptin receptor-deficient obese Zucker rats. Our findings reveal a novel hormonal crosstalk that rapidly modulates synaptic excitation via endocannabinoid release in the hypothalamus and that provides a nutritional state-sensitive mechanism to integrate the neuroendocrine regulation of energy homeostasis, fluid balance, and the stress response.

Most neurons in the nucleus tractus solitarii do not send collateral projections to multiple autonomic targets in the rat brain

The nucleus tractus solitarii (NTS) receives primary visceral afferents and sends projections to other autonomic nuclei at all levels of the neuroaxis. However, it is unknown if distinct populations of NTS neurons project to individual autonomic targets or if individual neurons in the NTS project to multiple autonomic targets. Understanding the basic circuitry of visceral reflex pathways is essential for the analyses of functional central autonomic networks. We examined projections from the NTS to autonomic targets within the hypothalamus (paraventricular nucleus, PVN), pons (parabrachial nucleus, PB), and medulla (caudal ventrolateral medulla, CVL) using retrograde tracing and immunohistochemistry. Dual retrograde tracer microinjections were made into pairs of targets (PVN + CVL; PVN + PB; PB + CVL), and the pattern of retrograde labeling was examined within NTS. The extent of collateralization, seen as dual retrogradely labeled neurons, was negligible for combined PVN and CVL injections and increased for injections combining PB with either PVN or CVL, but the majority of NTS neurons project to only one autonomic target. Immunohistochemistry for tyrosine hydroxylase (TH) was used to examine the pattern of TH-immunoreactivity (TH-ir) within retrogradely labeled NTS neurons. TH-ir was seen predominantly in projections to PVN, to a lesser degree in projections to PB, and was largely absent from projections to CVL. The percentage of dual retrogradely labeled neurons displaying TH-ir corresponded to the target displaying the most TH-ir, and TH-ir was not predictive of collateralization. Together, these results indicate that NTS neurons project to individual autonomic targets in the brain.

Electrophysiological properties and thermosensitivity of mouse preoptic and anterior hypothalamic neurons in culture

Responses of mouse preoptic and anterior hypothalamic neurons to variations of temperature are key elements in regulating the setpoint of homeotherms. The goal of the present work was to assess the relevance of culture preparations for investigating the cellular mechanisms underlying thermosensitivity in hypothalamic cells. Our working hypothesis was that some of the main properties of preoptic/anterior hypothalamic neurons in culture are similar to those reported by other authors in slice preparations. Indeed, cultured preoptic/anterior hypothalamic neurons share many of the physiological and morphological properties of neurons in hypothalamic slices. They display heterogenous dendritic arbors and somatic shapes. Most of them are GABAergic and their activity is synaptically driven by the activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate receptors. Active membrane properties include a depolarizing "sag" in response to hyperpolarization, and a low threshold spike, which is present in a majority of cells and is generated by T-type Ca2+ channels. In a fraction of the cells, the low threshold spike repeats rhythmically, either spontaneously, or in response to depolarization. The background synaptic noise in cultured neurons is characterized by the presence of numerous postsynaptic potentials which can be easily distinguished from the baseline, thus providing an opportunity for assessing their possible roles in thermosensitivity. An unexpected finding was that GABA-A receptors can generate both hyper- and depolarizing postsynaptic potentials in the same neuron. About 20% of the spontaneously firing preoptic/anterior hypothalamic neurons are warm-sensitive. Warming (32-41 degrees C) depolarizes some cells, a phenomenon which is Na+-dependent and tetrodotoxin-insensitive. The increased firing rate of warm-sensitive cells in response to warming can be prepotential and/or synaptically driven. Overall, our data suggest that a warm-sensitive phenotype is already developed in cultured cells. Therefore, and despite obvious differences in their networks, cultured and slice preparations of hypothalamic neurons can complement each other for further studies of warm-sensitivity at the cellular and molecular level.

Exercise Training Differentially Affects Intrinsic Excitability of Autonomic and Neuroendocrine Neurons in the Hypothalamic Paraventricular Nucleus

Oxytocinergic and vasopressinergic brain stem projections have been shown to play an important role in mediating cardiovascular adjustments during exercise training (ET). The aim of the present work was to determine whether the intrinsic excitability of hypothalamic neurons giving rise to brain stem peptidergic projections is altered as a consequence of ET. Whole cell patch-clamp recordings were obtained from nucleus of the solitarii tract (NTS)-projecting paraventricular nucleus of the hypothalamus (PVN) neurons and from supraoptic nucleus (SON) and PVN magnocellular cells (MNCs), in hypothalamic slices obtained from sedentary (S) and ET rats. Our results indicate that intrinsic excitability of PVN neurons that innervate the NTS (PVN-NTS) is enhanced by ET, resulting in a more efficient input-output function (increase number of evoked actions potentials, steeper frequency/current relationships and slower decaying frequency/time relationships). Changes in input-output function were accompanied by smaller hyperpolarizing afterpotentials (HAPs) and afterhyperpolarizing potentials (AHPs), during and after trains of spikes, respectively. On the other hand, a decreased efficacy in the input-output function was observed in SON/PVN MNCs during ET. Altogether, our results indicate that ET differentially affects the intrinsic excitability of autonomic and neurosecretory SON and PVN neurons. Increased excitability in PVN-NTS neurons may contribute to enhanced release of OT and VP peptides in the dorsal brain stem, and cardiovascular fine-tuning during exercise training.

Paraventricular nucleus, stress response, and cardiovascular disease

The paraventricular nucleus of the hypothalamus (PVN) is a complex effector structure that initiates endocrine and autonomic responses to stress. It receives inputs from visceral receptors, circulating hormones such as angiotensin II, and limbic circuits and contains neurons that release vasopressin, activate the adrenocortical axis, and activate preganglionic sympathetic or parasympathetic outflows. The neurochemical control of the different subgroups of PVN neurons is complex. The PVN has been implicated in the pathophysiology of congestive heart failure and the metabolic syndrome.

mu-opioid receptor mRNA expression in identified hypothalamic neurons

It has been known for a number of years that mu-opioid receptor agonists (e.g., morphine, beta-endorphin, and enkephalin) inhibit luteinizing hormone (LH), vasopressin (VP), and oxytocin (OT) release and stimulate prolactin secretion in rodents and primates by an action at the level of the brain. Also, electrophysiological studies have established that hypothalamic neurons, including gonadotropin-releasing hormone (GnRH), VP, OT, beta-endorphin, and dopamine neurons, are responsive to mu-receptor activation. Although mu-receptor expression has been demonstrated in the hypothalamus, there have been few studies localizing these receptors in neurosecretory neurons. Therefore, we sought to document mu-opioid receptor mRNA expression in immunocytochemically identified hypothalamic neurons. The brains from both female and male guinea pigs were examined by using in situ hybridization and immunocytochemistry. The studies revealed that mu-receptor mRNA was expressed in different diencephalic regions including the preoptic area, the bed nuclei stria terminalis, the paraventricular nucleus thalamus, and the anterior hypothalamus, as well as the supraoptic (SON), paraventricular (PVH), ventromedial, dorsomedial, and arcuate nuclei of the hypothalamus. Importantly, mu-opioid receptors were expressed in subpopulations of GnRH neurons (33.25 +/- 4.6% and 33.6 +/- 3.7% in females and males, respectively), dopamine neurons (51.7 +/- 5.8% to 75.0 +/- 2.6%, depending on neuronal location), beta-endorphin neurons (68.3.0 +/- 4.4%), and VP neurons (41-70%, depending on neuronal location). Because mu-opioid receptors couple via G-proteins to activate inwardly rectifying potassium channels and to inhibit calcium channels, the presence of these receptors is likely to play a major role in directly controlling the excitability of hypothalamic neurons.

Prostaglandin E2 mediates cellular effects of interleukin-1beta on parvocellular neurones in the paraventricular nucleus of the hypothalamus

Abstract Interleukin-1beta (IL-1beta) is involved in hypothalamic regulation of corticotrophin-releasing hormone secretion, autonomic activation and consequent downstream modulation of the neuroimmune response. Previously, we have shown that IL-1beta depolarises parvocellular neurones in the paraventricular nucleus (PVN) of the hypothalamus, and these effects are dependent on attenuation of gamma-amino butyric acid (GABA)-ergic input. In the present study, using whole-cell patch clamp recordings of rat neurones in a slice preparation of the PVN, we show that the effects of IL-1beta are abolished in the presence of a cyclooxygenase (COX)-2 inhibitor, NS-398, indicating a dependence on prostaglandin (PG) synthesis and activation. In response to 1 microM PGE2, 64% of parvocellular neurones tested exhibited a clear depolarisation, which was abolished in the presence of tetrodotoxin (TTX). Furthermore, neurones responsive to both IL-1beta and PGE2 exhibited a decrease in the frequency of inhibitory post-synaptic potentials, suggesting that effects of these modulators are mediated via a decrease in GABA-ergic input to these neurones. A proportion (44% and 40%, respectively) of putative GABA-ergic neurones in the halo region surrounding the PVN demonstrated hyperpolarising responses to 1 nM IL-1beta and 1 microM PGE2, and these effects were maintained in TTX. Furthermore, direct hyperpolarising effects of IL-1beta were blocked in the presence of NS-398. Together, these data suggest that PGE2, synthesised in response to IL-1beta-activation of COX-2 expressing cells, directly hyperpolarises putative GABA-ergic neurones in the halo zone surrounding and projecting to the PVN, resulting in a decrease in GABA-ergic input to parvocellular neurones and consequent depolarisation. These data further elucidate the cellular mechanisms by which IL-1beta exerts its neuroimmune-related actions in the PVN.

Angiotensin depolarizes parvocellular neurons in paraventricular nucleus through modulation of putative nonselective cationic and potassium conductances

Neurosecretory parvocellular neurons in the hypothalamic paraventricular nucleus (PVN) exercise considerable influence over the adenohypophysis and thus play a critical role in neuroendocrine regulation. ANG II has been demonstrated to act as a neurotransmitter in PVN, exerting significant impact on neuronal excitability and also influencing corticotrophin-releasing hormone secretion from the median eminence and, therefore, release of ACTH from the pituitary. We have used whole cell patch-clamp techniques in hypothalamic slices to examine the effects of ANG II on the excitability of neurosecretory parvocellular neurons. ANG II application resulted in a dose-dependent depolarization of neurosecretory neurons, a response that was maintained in tetrodotoxin (TTX), suggesting a direct mechanism of action. The depolarizing actions of this peptide were abolished by losartan, demonstrating these effects are AT(1) receptor mediated. Voltage-clamp analysis using slow voltage ramps revealed that ANG II activates a voltage-independent conductance with a reversal potential of -37.8 +/- 3.8 mV, suggesting ANG II effects on a nonselective cationic current. Further, a sustained potassium current characteristic of I(K) was significantly reduced (29.1 +/- 4.7%) by ANG II. These studies identify multiple postsynaptic modulatory sites through which ANG II can influence the excitability of neurosecretory parvocellular PVN neurons and, as a consequence of such actions, control hormonal secretion from the anterior pituitary.

Actions of neuropeptide W in paraventricular hypothalamus: implications for the control of stress hormone secretion

Neuropeptide W (NPW) is produced in neurons located in hypothalamus and brain stem, and its receptors are present in the hypothalamus, in particular in the paraventricular nucleus (PVN). Intracerebroventricular (ICV) administration of NPW activated, in a dose-related fashion, the hypothalamic-pituitary-adrenal axis, as determined by plasma corticosterone levels in conscious rats but, at those same doses, did not stimulate the release of oxytocin or vasopressin into the peripheral circulation or alter blood pressure or heart rate. The ability of ICV-administered NPW to stimulate the hypothalamic-pituitary-adrenal axis in conscious male rats was blocked by intravenous pretreatment with a corticotropin-releasing hormone antagonist. This suggested an action of NPW in the parvocellular division of the PVN. Indeed, in hypothalamic slice preparations (whole cell patch recording), bath application of NPW depolarized and increased the spike frequency of the majority of electrophysiologically identified putative neuroendocrine PVN neurons. Effects on membrane potential were maintained in the presence of TTX, suggesting them to be direct postsynaptic actions on these neuroendocrine cells. Our data suggest that endogenous NPW, produced in brain, may play a physiologically relevant role in the neuroendocrine response to stress.

A possible mechanism for the action of adrenomedullin in brain to stimulate stress hormone secretion

Adrenomedullin (AM) has been reported to have actions at each level of the hypothalamo-pituitary-adrenal (HPA) axis, suggesting that the peptide plays a role in the organization of the neuroendocrine responses to stress. We examined the mechanism by which AM regulates the central nervous system branch of the HPA axis as well as the possible role of AM in the modulation of the releases of two other hormones, prolactin and GH, whose secretions also are altered by stress. Intracerebroventricular administration of AM led to elevated plasma corticosterone levels in unrestrained, conscious male rats. This effect was abrogated by pretreatment with a CRH antagonist, suggesting that AM activates the HPA axis by causing the release of CRH into hypophyseal portal vessels. In addition, AM given intracerebroventricularly stimulated the release of prolactin but did not alter the secretion of GH. We propose that AM produced in the brain may be an important neuromodulator of the hormonal stress response.

Morphology and electrophysiology of neurons in dog paraventricular nucleus: in vitro study

The paraventricular nucleus (PVN) of the hypothalamus plays an important role in regulating gut motility. To date, there have been no intracellular electrophysiological studies of dog PVN neurons in vitro. The aims of this study were to: (1) adapt brain slice methods developed for studies of rodent CNS tissue to canine CNS tissue; and (2) study the electrophysiology and morphology of single neurons of the dog paraventricular nucleus (PVN). Coronal hypothalamic slice preparations (400 microm thick) of dog brain were used. Three groups of PVN neurons were classified based on their firing pattern. Continuous firing neurons (n=32) exhibited continuous ongoing action potentials (APs). Burst firing neurons generated bursts of APs (n=19). Intermittent firing neurons had only a few spontaneous APs. In contrast to continuous firing neurons, 14 of 19 burst firing neurons and 3 of 7 intermittent firing neurons responded to depolarizing current with a Ca2+-dependent low-threshold potential. Twenty-one PVN neurons studied electrophysiologically were filled with biocytin. Continuous firing neurons (n=12) had oval-shaped soma with two or three sparsely branched dendrites. Branched axons were found in two continuous firing neurons, in which one branch appeared to terminate locally. Burst firing neurons (n=8) generally had triangular soma with 2 to 5 branched dendrites. In summary, the brain slice technique was used to study the morphology and electrophysiology of single neurons of the dog brain. Electrophysiological and morphological properties of the three neuron groups were identified and discussed.

Nitric oxide and homeostatic control: an intercellular signalling molecule contributing to autonomic and neuroendocrine integration?

Accumulated evidence indicates that nitric oxide (NO) plays a pivotal role in the central control of bodily homeostasis, including cardiovascular and fluid balance regulation. Two major neuronal substrates mediating NO actions in the control of homeostasis are the paraventricular nucleus (PVN) of the hypothalamus, considered a key center for the integration of neuroendocrine and autonomic functions, and the supraoptic nucleus (SON). In this work, a comprehensive review of NO modulatory actions within the SON/PVN, including NO actions on neuroendocrine and autonomic outputs, as well as the cellular mechanisms underlying these effects is provided. Furthermore, this review comprises recent progress from our laboratory that adds to our current understanding of the cellular sources, targets and mechanisms underlying NO actions within neuroendocrine and autonomic hypothalamic neuronal circuits. By combining in vitro patch clamp recordings, tract-tracing neuroanatomy, immunohistochemistry and live imaging techniques, we started to shed light into the cellular sources and signals driving NO production within the SON and PVN, as well as NO actions and mechanisms targeting discrete neuronal populations within these circuits. Based on this new information, we have expanded one of the current working models in the field, highlighting a key role for NO as a signaling molecule that facilitates crosstalk among various cell types and systems. We propose that this dynamic NO signaling mechanisms may constitute a neuroanatomical and functional substrate underlying the ability of the SON and PVN to coordinate complex neuroendocrine and autonomic output patterns.

Nitric oxide inhibits the firing activity of hypothalamic paraventricular neurons that innervate the medulla oblongata: role of GABA

Nitric oxide (NO) has been shown to modulate autonomic function by acting both peripherally and centrally. A growing body of evidence indicates that the paraventricular nucleus of the hypothalamus (PVN), an important site for autonomic and endocrine homeostasis, constitutes an important locus mediating central NO actions. However, the cellular targets and mechanisms mediating NO actions within the PVN are not completely understood. Here, we examined whether NO influences the firing activity of identified PVN neurons that innervate two functionally different autonomic centers, the dorsal vagal complex (DVC) and the rostral ventrolateral medulla (RVLM). Perforated patch-clamp recordings were performed in hypothalamic slices containing retrogradely labeled PVN neurons innervating the DVC or the RVLM. Application of the NO donors dyethylamine- or 1-propanamine, 3-(2-hydroxy-2-nitroso-1-propylhydrazino) NONOate inhibited the firing activity of both DVC- and RVLM-projecting PVN neurons. Furthermore, application of 2-(4-carboxypheny)-4,4,5,5,-tetramethilimidazoline-1-oxyl-3-oxide (carboxy-PTIO), or the relatively selective neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole alone, increased their basal firing activity, suggesting the presence of an endogenous NO inhibitory tone. GABAergic synaptic activity in PVN neurons was potentiated by NO donors, an action that involved a presynaptic mechanism. Furthermore, the NO-mediated inhibition of firing activity was blocked by the GABA(A) receptor antagonist bicuculline, suggesting that NO-inhibitory actions involved potentiation of local GABAergic synaptic activity. Immunohistochemical studies showed that approximately 25% of DVC- and RVLM-projecting PVN neurons express nNOS, suggesting that a proportion of these medullary-projecting PVN neurons contribute to the cellular source of NO within the PVN. In summary, NO has been identified as an important molecule controlling autonomic function under physiological and pathological conditions. Here, we provide information on the cellular mechanisms mediating central NO actions. Our results demonstrate for the first time that NO modulates the activity of identified populations of PVN neurons that innervate the medulla oblongata, an action that is likely mediated by enhancing synaptic GABAergic function. This work suggests that NO-GABA interaction in PVN neurons that innervate the medulla constitutes an efficient cellular mechanism mediating NO central regulation of autonomic function.

Neuromedin U depolarizes rat hypothalamic paraventricular nucleus neurons in vitro by enhancing IH channel activity

The effect of neuromedin U (NMU) on rat paraventricular nucleus (PVN) neurons was examined using whole cell patch-clamp recordings. Under current-clamp, 31% of PVN parvocellular neurons (n = 243) were depolarized by 100 nM NMU, but magnocellular neurons were not affected. NMU (10 nM to 1 microM) resulted in increased basal firing rate and depolarization in a dose-dependent manner with an EC50 of 70 nM. NMU-induced depolarization was unaffected by co-perfusion with 0.5 microM TTX + 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + 10 microM bicuculline. Extracellular application of 70 microM ZD 7288 completely inhibited NMU-induced depolarization. Under voltage-clamp, 1 microM NMU produced negligible inward current but did increase the hyperpolarization-activated current (IH) at step potentials less than -80 mV. The effects of NMU on IH were voltage-dependent, and NMU shifted the IH conductance-voltage relationship (V1/2) by about 10.8 mV and enhanced IH kinetics without changing the slope constant (k). Extracellular application of 70 microM ZD 7288 or 3 mM Cs+ blocked IH and the effects of NMU in voltage-clamp. These results suggest that NMU selectively depolarizes the subpopulation of PVN parvocellular neurons via enhancement of the hyperpolarization-activated inward current.

Fos, RVLM-projecting neurons, and spinally projecting neurons in the PVN following hypertonic saline infusion

Hypertonic saline (HTS; 1.7 M) infused intravenously into conscious rats increases the production of Fos, a marker of cell activation, in the hypothalamic paraventricular nucleus (PVN). The parvocellular PVN contains subpopulations of neurons. However, which subpopulations are activated by HTS is unknown. We determined whether PVN neurons that innervate the rostral ventrolateral medulla (RVLM) or the spinal cord (important autonomic sites) expressed Fos following HTS. Experiments were performed 24-96 h after chronic implantation of an intravenous cannula. HTS significantly increased the number of Fos-positive cells. In the parvocellular PVN, the maximum number of Fos-positive cells occurred rostral of the anterior-posterior level at which the number of neurons that projected to the medulla or spinal cord peaked. Compared with controls, HTS did not significantly increase the number of double-labeled neurons. These findings demonstrate that an elevation in plasma osmolality activates PVN neurons but not the subgroups of PVN neurons with projections to the RVLM or to the spinal cord.

Cardiovascular effects of leptin and orexins

Leptin, the product of the ob gene, is a satiety factor secreted mainly in adipose tissue and is part of a signaling mechanism regulating the content of body fat. It acts on leptin receptors, most of which are located in the hypothalamus, a region of the brain known to control body homeostasis. The fastest and strongest hypothalamic response to leptin in ob/ob mice occurs in the paraventricular nucleus, which is involved in neuroendocrine and autonomic functions. On the other hand, orexins (orexin-A and -B) or hypocretins (hypocretin-1 and -2) were recently discovered in the hypothalamus, in which a number of neuropeptides are known to stimulate or suppress food intake. These substances are considered important for the regulation of appetite and energy homeostasis. Orexins were initially thought to function in the hypothalamic regulation of feeding behavior, but orexin-containing fibers and their receptors are also distributed in parts of the brain closely associated with the regulation of cardiovascular and autonomic functions. Functional studies have shown that these peptides are involved in cardiovascular and sympathetic regulation. The objective of this article is to summarize evidence on the effects of leptin and orexins on cardiovascular function in vivo and in vitro and to discuss the pathophysiological relevance of these peptides and possible interactions.

Nitric oxide: a local signalling molecule controlling the activity of pre-autonomic neurones in the paraventricular nucleus of the hypothalamus

AIM

The gas molecule nitric oxide (NO) has been shown to modulate autonomic function by acting both peripherally and centrally. Accumulating evidence indicates that the paraventricular nucleus (PVN) of the hypothalamus is an important locus mediating central NO actions on autonomic function, under both physiological and pathological conditions. However, the cellular targets and mechanisms mediating NO actions within the PVN are still poorly understood.

RESULTS

By combining in vitro patch-clamp recordings with neuronal tract tracing techniques, we show that neuronal excitability of autonomic-related neurones in the PVN is tonically inhibited by an endogenous NO input. Furthermore, immunohistochemical studies show that approximately 25% of autonomic-related PVN neurones express neuronal nitric oxide synthase, suggesting that at least a proportion of them contribute to the cellular sources of NO within the PVN.

CONCLUSION

In summary, this work suggests that NO modulation of the firing activity of autonomic-related PVN neurones constitutes an efficient mechanism mediated central NO regulation of autonomic function.

Suprachiasmatic control of melatonin synthesis in rats: inhibitory and stimulatory mechanisms

The suprachiasmatic nucleus (SCN) controls the circadian rhythm of melatonin synthesis in the mammalian pineal gland by a multisynaptic pathway including, successively, preautonomic neurons of the paraventricular nucleus (PVN), sympathetic preganglionic neurons in the spinal cord and noradrenergic neurons of the superior cervical ganglion (SCG). In order to clarify the role of each of these structures in the generation of the melatonin synthesis rhythm, we first investigated the day- and night-time capacity of the rat pineal gland to produce melatonin after bilateral SCN lesions, PVN lesions or SCG removal, by measurements of arylalkylamine N-acetyltransferase (AA-NAT) gene expression and pineal melatonin content. In addition, we followed the endogenous 48 h-pattern of melatonin secretion in SCN-lesioned vs. intact rats, by microdialysis in the pineal gland. Corticosterone content was measured in the same dialysates to assess the SCN lesions effectiveness. All treatments completely eliminated the day/night difference in melatonin synthesis. In PVN-lesioned and ganglionectomised rats, AA-NAT levels and pineal melatonin content were low (i.e. 12% of night-time control levels) for both day- and night-time periods. In SCN-lesioned rats, AA-NAT levels were intermediate (i.e. 30% of night-time control levels) and the 48-h secretion of melatonin presented constant levels not exceeding 20% of night-time control levels. The present results show that ablation of the SCN not only removes an inhibitory input but also a stimulatory input to the melatonin rhythm generating system. Combination of inhibitory and stimulatory SCN outputs could be of a great interest for the mechanism of adaptation to day-length (i.e. adaptation to seasons).

Neurosecretory and non-neurosecretory parvocellular neurones of the hypothalamic paraventricular nucleus express distinct electrophysiological properties

Parvocellular neurones of the hypothalamic paraventricular nucleus (PVN) comprise neurosecretory and non-neurosecretory subpopulations. We labelled neurosecretory neurones with intravenous injection of the retrograde tracer, fluoro-gold, and recorded from fluoro-gold-positive and negative PVN parvocellular neurones in hypothalamic slices. Non-neurosecretory parvocellular neurones generated a low-threshold spike (LTS) and robust T-type Ca2+ current, whereas neurosecretory neurones showed no LTS and a small T-current. LTS neurones were located in non-neurosecretory regions of the PVN, and non-LTS neurones were located in neurosecretory regions of the PVN. These findings indicate that neurosecretory and non-neurosecretory subtypes of parvocellular PVN neurones express distinct membrane electrical properties.